1. Field of the Invention
The present invention relates to a radar for detecting a vehicle, etc. using, for example, electromagnetic waves in the millimeter-wave band.
2. Description of the Related Art
Generally, the strength S of a reception signal received by a radar varies in accordance with distance. The relationship between the strength S of a signal received by a radar and the distance can be expressed by the following radar equation:
S=(PG2xcex2"sgr")/((4xcfx80)3R4xc3x97L) 
where P denotes the output power, G denotes the antenna gain, xcex denotes the wavelength, "sgr" denotes the scattering cross-section, R denotes the distance, and L denotes the propagation loss.
Thus, the strength S of a signal received by a radar is inversely proportional to R4; therefore, when the radar is used over a large range from a short distance to a long distance, the strength S varies in a wide range in accordance with the distance of a target from an antenna.
A beat signal obtained by mixing a reception signal and a transmission signal provides an intermediate-frequency signal (IF signal), and the strength of the IF signal is proportional to that of the reception signal. In order to allow detection at a long distance, the gain of an amplifier circuit for amplifying the IF signal must be increased in order to compensate for the decrease in the strength of the reception signal. However, if the gain of the amplifier circuit is too large, when a signal received from a short distance is amplified, the amplified signal exceeds the input range of an analog to distal (AD) converter, causing an overflow.
In order to prevent the overflow of the AD converter, proposals have been disclosed including (1) Japanese Unexamined Patent Application Publication No. 7-151851 in which an amplifier circuit has an AGC (automatic gain control) capability; (2) Japanese Unexamined Patent Application Publication No. 8-334557 in which an antenna has an AGC capability; and (3) Japanese Unexamined Patent Application Publication No. 8-211144 in which an amplifier circuit or a transmission circuit has an AGC capability.
However, such an AGC capability can only be implemented by adding complex circuitry, increasing the overall size and incurring additional cost. Furthermore, because AGC is usually achieved by a loop or feedback control, a sufficient AGC is not achieved against an instantaneous change in the strength of reception signal due to the effect of the time constant of the control loop.
Other proposals include:
(4) Japanese Unexamined Patent Application Publication No. 10-142322 and (5) Japanese Unexamined Patent Application Publication No. 7-77575 which each disclose a radar which generates a frequency-modulated transmission signal whose frequency varies with time, and which detects a distance to a target based on the beat frequency, wherein the gain of an amplifier circuit is varied in accordance with the beat frequency, considering that the strength of a signal received from a long distance is weak.
Generally, the level of a noise signal generated within an amplifier increases in proportion to the passband width of the amplifier. An amplifier circuit for amplifying an IF signal in a radar requires a high SN ratio in order to amplify a weak signal. However, if the gain of an amplifier increases as the frequency of the input signal increases, as in (4) and (5) described above, because the level of a noise signal in an output signal of the amplifier is proportional to the square root of the passband width, the noise signal increases and the SN ratio is thus degraded.
The amplified IF signal is converted into digital data in an AD converter. If a frequency component at or above one half of the sampling frequency is present in an input signal to the AD converter, a problem occurs. That is, the frequency components at or above one half of the sampling frequency are folded over and superposed on the frequency components lower than the center frequency of the sampling frequency, causing a detection of a false image.
Furthermore, in the lower range of IF frequencies corresponding to short distances, the gain is decreased, inhibiting detection of small targets.
Furthermore, the radars disclosed in (4) and (5) do not specify how to handle DC components and low-frequency components in the vicinity of DC in the IF signal. With regard to the DC component and low-frequency components in the vicinity of DC in the IF signal, there have been problems to be solved as follows.
To a local signal input terminal of a mixer, a weak transmission signal (a leakage signal from a circulator) is provided, and to an RF signal input terminal of the mixer, a weak transmission signal Tm reflected or transmitted within the radar apparatus, as well as a reception signal S from a target, are provided. When compared in terms of power level, reflection by an antenna radiator is usually much larger than propagation loss, and thus Tm is considerably larger than S.
When a weak transmission signal is input to the local signal input terminal of the mixer and Tm is input to the RF input terminal of the mixer as described above, it is equivalent to the input of a reflection signal from a very short distance, generating a DC component.
Furthermore, a high-frequency signal generated by a voltage-controlled oscillator VCO for generating a transmission signal causes noise signals in the vicinity of the carrier frequency, due to thermal noise, flicker noise in semiconductors, etc. These types of noise signals are referred to as a sideband noise signal and a phase noise signal.
If a frequency-modulated signal including the sideband noise signal or the phase noise signal is input to the local signal input terminal or the RF signal input terminal of the mixer, a noise signal is generated in the vicinity of DC.
The DC and the noise signal in the vicinity of DC, superposed on the IF signal, may reduce the sensitivity of the radar, degrade SN ratio, and cause an erroneous detection.
Accordingly, it is an object of the present invention to provide a radar in which the problems described above are overcome by determining the frequency characteristics of an amplifier circuit for amplifying an IF signal as appropriate.
The present invention, in one aspect thereof, provides a radar for detecting the distance to a target based on the frequency difference between a transmission signal and a reception signal. The radar includes a transmission circuit for generating a frequency-modulated transmission signal whose frequency varies in time; a mixer circuit for generating an IF signal representing the frequency difference between a reception signal and the transmission signal; an amplifier circuit for amplifying the IF signal, the gain thereof having its peak at or below one half of a sampling frequency of an AD converter coupled to the output of the amplifier circuit, preferably at the frequency of an IF signal corresponding to a maximum detection distance; and the AD converter sampling the IF signal at a predetermined sampling frequency and converting from analog to digital. The maximum detection distance means the longest detection distance set in or expected for the radar.
Accordingly, the frequency range of a signal which is input to the amplifier circuit is restricted, so that the SN ratio is improved.
The present invention, in another aspect thereof, provides a radar for detecting the distance to a target based on the frequency difference between a transmission signal and a reception signal. The radar includes a transmission circuit for generating a frequency-modulated transmission signal whose frequency varies in time; a mixer circuit for generating an IF signal representing the frequency difference between a reception signal and the transmission signal; an amplifier circuit for amplifying the IF signal; a DC blocking circuit provided at the input of the amplifier circuit; and an offset circuit for adding a predetermined DC offset, provided at the output of the amplifier circuit.
Accordingly, undesirable effects of the DC component and noise signal components in the vicinity of DC, superposed on the IF signal, are avoided, preventing reduction in sensitivity, degradation of SN ratio, and erroneous detection.
The present invention, in yet another aspect thereof, provides a radar for detecting the distance to a target based on the frequency difference between a transmission signal and a reception signal. The radar includes a transmission circuit for generating a frequency-modulated transmission signal whose frequency varies in time; a mixer circuit for generating an IF signal representing the frequency difference between a reception signal and the transmission signal; an amplifier circuit for amplifying the IF signal, the gain thereof having its peak at or below one half of a sampling frequency of an AD converter coupled to an output of the amplifier circuit; the AD converter sampling the IF signal at a predetermined sampling frequency and converting from analog to digital; a first DC blocking circuit provided at the input of the amplifier circuit; a second DC blocking circuit provided at the output of the amplifier circuit; and an offset circuit, provided at the output of the amplifier circuit, for adding a predetermined DC offset to a signal from which the DC component has been removed by the second DC cutting circuit.
The gain of the amplifier circuit preferably increases as the frequency rises in a range of at or below one half of the sampling frequency, preferably in a frequency range of the IF signal corresponding to distances not longer than a maximum detection distance.
Accordingly, as opposed to the related art, complex circuitry such as an AGC circuit is not required, serving to reduce the overall size and cost. Furthermore, unlike AGC circuits, an instantaneous change in the strength of the reception signal can be properly dealt with.
In a frequency range of the IF signal in which the output signal of the mixer saturates as the distance to the target becomes shorter, within a maximum allowable range of input voltage to an AD converter for converting an output signal from the amplifier circuit into digital data, the ratio of change in the gain of the amplifier relative to change in the frequency is preferably made smaller.
Accordingly, the gain at short distances are made relatively larger so as to improve sensitivity, whereby the problem of relative insufficiency in sensitivity at short distances due to a saturation of the mixer is prevented. Thus, a target at a short distance can be accurately detected.